just use the "view rendered source" Chrome extension.
Yes, that works.
Or you can right click and select "inspect element", and all the rendered code is displayed (an updated as you use the page), no browser extension needed. Also works, on Edge, Opera, Brave, Firefox, Safari, and IE.
There are also bonus features:
Console tab: shows any messages logged to the console, including warnings and errors.
Source tab: Let's you see the source code of the Javascript programs running on the page.
Network tab: See every network request made by the page, including tracking request, websockets, and all the request and response headers.
and more tabs that I wont get into.
It's all right there in your plain vanilla browser. It is very powerful stuff, not just for "spying" on other websites but also for making sure your website does what you expect it to.
Sorry for the rant!